1. Field of the Invention:
This invention relates to key telephone systems and, more particularly, to a key telephone system capable of operating in modes such as hand-free talk-back, off-hook signaling and the like with a less number of wiring cables between a main equipment and respective key telephones.
What the present invention further relates to is more specifically, a key telephone system wherein the number of wiring cables between the main equipment and each key telephone is reduced to six, with which two sets of two-way voice frequency talking paths, one set of two-way digital data transmitting paths and electric power supplying paths from the main equipment to the respective key telephones are formed. The invention also pertains to a key telephone system wherein a plurality of key telephones are provided with two sets of voice frequency talking circuits connected respectively to the two sets of two-way voice frequency talking paths, and the main equipment is provided with a key telephone interface circuit corresponding to each of the key telephones and connected to the two sets of the two-way voice frequency talking paths. The main equipment also includes a plurality of switching matrixes connecting the key telephone interface circuit to an interface circuit of central office lines (which shall be referred to as CO lines hereinafter), an intercom trunk and audible signal source, and a further plurality of switching matrixes connecting the CO line interface circuit to an audible signal source and a circuit for sending dual-tone multifrequency signals (which shall be referred to as DTMF signals hereinafter). The invention further relates to a key telephone system wherein the DTMF signal generating circuit is eliminated from each of the plurality of key telephones and the dial digit signal of each key telephone is transmitted to the main equipment to be converted to a DTMF signal by using a DTMF signal of a DTMF sending circuit in the main equipment and then sent to the CO lines (subscriber's lines) through the CO line interface circuit.
2. Description of Prior Art:
In conventional key telephone systems in which the number of wiring cables between the key telephone and the main equipment is six, two of the wiring cables are used as a voice frequency talking path, another two are used as digital data transmitting paths from the main equipment to the key telephone, and the remaining two are used as digital data transmitting paths from the key telephone to the main equipment. In these systems, the voice frequency talking path is single and there has been a problem in that the key telephone responding to one incoming call can not respond to another incoming call requirement.
It has become increasingly necessary, therefore, to solve such problem, by means of, for example, an off-hook signaling function provided to the key telephone for responding to another incoming call requirement with a speaker and microphone arranged in the key telephone during use of an ordinary handset so as to immediately deal with congested talking requirements, that is, to provide a key telephone system of a high efficiency capable of responding to as many incoming call requirements as possible without blocking an ongoing conversation. In order to respond to this requisite, two further wiring cables have been additionally arranged between the key telephone and the main equipment so as to form a second voice frequency talking path. In this case, however, there have been still involved such problems that, as the number of the wiring cables increases, it is necessary to secure spaces for housing the increased number of cables in the main equipment and key telephone, required wiring work becomes complicated and, as a result, it is difficult to save required materials and to lower the costs. Further, due to the additional arrangement of a limited number of the wiring cables, the requisite of increasing the talking efficiency has not been well satisfied. The present invention has been suggested in view of these problems.
A primary object of the present invention is, therefore, to provide a novel key telephone system wherein the number of wiring cables is maintained at six, wherein two of the two-way voice frequency talking paths are formed of four of the six wiring cables, a two-way digital data transmitting path is formed of the remaining two of the wiring cables, and the two wiring cables forming one of the two talking paths and the two wiring cables forming the data transmitting path are respectively utilized as phantom lines to form electric power supplying paths.
Another object of the present invention is to provide a key telephone system provided with a plurality of switching matrixes for utilizing two of the two-way voice frequency talking paths to implement the functions of handset talking, speaker talking, intercom hand-free talking, off-hook signaling and tone signaling.
A further object of the present invention is to provide a key telephone system wherein the two-way digital data transmitting paths are efficiently utilized so as to eliminate the costly tone generating circuit from the DTMF signal generating dial circuit of the key telephone.
Yet another object of the present invention is to provide a key telephone system provided with a plurality of matrixes connecting a pair of two-way voice frequency circuits and a further plurality of switching matrixes connecting a two-way digital data transmitting path to such signal source as a DTMF sending circuit or the like so as to remarkably improve the respective functions of the key telephone at low manufacturing costs.